In present day pressurized water reactors (PWRs), the reactivity is controlled mainly with control clusters each comprising a bundle of elongated elements suspended from a spider with radial arms carried by a vertical cluster drive shaft, for moving the cluster between a lower position in which it is inserted almost completely in the core of the reactor and an upper position in which it is situated in the upper internal equipment of the reactor.
The elements are guided in the core by guide tubes provided in the fuel assemblies. Above the core, these elements, which are very long and thin, must be guided in the upper internals by a device which may comprise: a tubular casing, having, at its upper part, a closure plate formed with a hole for passing the drive shaft therethrough; horizontal plates spaced apart along the casing and cut out to let the spider pass and to guide the elements; means for continuously guiding the elements in the low part of the casing, formed by guide tubes for individual elements and/or by guide sleeves for pairs of elements each having a slit of sufficient width to let an arm of the spider pass, in the intervals between the lowest plates, intended to protect the elements from the action of the turbulent flow of coolant which leaves the core and is deflected towards coolant outlet openings formed in the casing and situated in said intervals and to avoid buckling of the elements when the cluster is released and falls while being braked by the dash-pot effect of the guide tubes.
Up to now, attempts have been made to reduce as much as possible the pressure gradients which are exerted on the elements in a horizontal direction, in the above-mentioned intervals. Such attempts have been based on the assumption that such pressure gradients apply the elements against the lips of the slit through which the spider passes and cause frictional wear of the elements. To achieve pressure balancing, pressure equalizing holes have been formed in the wall of the guide sleeves; the sleeves have been replaced by castings leaving a pressure balancing clearance (U.S. Pat. No. 5,006,305); and indentations have been formed in the lips of the slit which constitute pressure balancing openings (U.S. Pat. No. 4,562,038). The subjects matter of such patents are included in the present specification by reference. Other examples of prior art guiding devices may be found in French Patent No. 2 397 043 (WESTINGHOUSE) and European No. 329,536 (FRAMATOME).